This invention relates to devices for switching high voltage d-c power, and more specifically relates to a novel high voltage switching device using a magnetic modulated vacuum arc discharge. Devices made in accordance with the invention have application, typically, as a transfer switching element for a fault current limiter in an a-c power transmission system, and also have application as a high voltage d-c circuit breaker, as an inverter element and as a switching element for magnetic energy storage systems.
Arrangements for switching high voltage d-c using a magnetically modulating vacuum arc discharge is known, and is described, for example, in the IEEE Transactions on Electron Devices, Volume ED-22, No. 4, April 1975, pages 173 to 180 in a paper entitled THE INTERRUPTION OF VACUUM ARCS AT HIGH DC VOLTAGES, by Alexander S. Gilmour, Jr. and David L. Lockwood. Such devices are also described in U.S. Pat. No. 3,696,264, dated Oct. 3, 1972 in the name of Clark et al. In the arrangement shown in the above paper, a cathode is disposed on the axis of a ring-shaped anode element. The anode is surrounded by a magnetic field-producing coil. This entire assemblage is then mounted within a vacuum container. The paper suggests that it might be possible to put the coil outside of the vacuum envelope. The cathode serves as a source of electrons which flow to the surrounding anode. The arc current is controlled and extinguished by producing a magnetic field from the coil, which magnetic field passes through the arc path. The magnetic field is produced by current through the coil. The duration and amplitude of the magnetic field can then be controlled sufficiently to cause extinction of arc current flow. When arc conduction ceases, the metal vapor from the cathode which provided the arc current medium rapidly condenses and the circuit between the device terminals is opened. Experimental devices have been built with the capability to interrupt a direct current of 800 amperes at 25 kV and 8 kA at a recovery voltage of 8 kV. These parameters were determined by the experimental setup. Higher capability is possible, depending on the component design.
The arrangement shown in the paper by Gilmour et al has several shortcomings which would prevent its use as a commercial device. For example, in order to make connection to the anode in Gilmour's arrangement, it is necessary to provide high voltage vacuum feed-through terminals which pass through the vacuum container and are connected to the anode ring. These high voltage feed-throughs are particularly troublesome components and are a source of frequent failure.
Another difficulty in the arrangement proposed by Gilmour et al is that the power for the magnetic control field is supplied from a power supply with an electronically controlled time delay or by an anode coil in which the discharge current produces a magnetic field essentially without time delay. When this device is to be applied as a fault current limiter switching element, a controlled time delay is needed for the magnetic field with respect to the arc discharge current. The present invention provides a novel arrangement consisting of radially disposed metal plates enclosed by the anode for delaying the magnetic field buildup through the design of the anode, thus eliminating the need for a separate power supply and electronics for producing the magnetic field.
A further difficulty with the device proposed by Gilmour et al is that the rate of change of current during switching is extremely high so that the device will generate extensive surge voltages in inductive networks.